Desalting of crude oil before it enters the various refinery processing units has been a common practice for many years. It is a purifying process to remove water, salts, silts, clays and other inorganic materials such as rust, iron sulfide, etc. Through the removal of such materials, a cleaner crude oil is charged to the refinery processing units. This operation prevents or reduces costly problems due to corrosion, fouling of the refinery equipment and accelerated catalyst poisoning. Higher quality products are also produced. Other benefits from desalting are less frequent turnarounds and savings in chemical treatment.
Until the recent years, crude desalting in plant units was a rather routine and simple operation. A relatively small amount of water was mixed well with the crude and the emulsion separated in a vessel using an electrostatic field to accelerate the coalescence of the water. Demulsifiers were used on crudes that possessed some emulsion stability. The desalted crude was sent to the downstream units for processing to quality products. Generally desalters worked well, required little attention and were almost a forgotten unit. This situation has changed.
Two factors affecting desalter operation have entered the scene in the last few years. The first relates to chemicals added to crude oil. Certain producers of crude oil are now using what is considered third-generation technology to further extract oil from oil-producing formations. Various chemicals, such as sulfonates, are employed to recover this tertiary oil. While producers have some concern regarding chemicals they use and their effect on refinery processing, their primary objective is to produce more oil. Processing problems, then, fall upon the refiners to handle. Although tertiary crude production has been relatively small compared to primary and secondary production, refiners have been burdened with some thorny problems by this oil.
The second factor relating to difficulties in plant desalter operation concerns processing heavier crudes. Generally, process desalting of crudes of about 30.degree.API and lighter presents little or no problems. As crudes get heavier (particularly 20.degree.API and heavier), water separation is slowed and producing a clean desalted product can be difficult. The settling rate of water droplets in a desalter follows Stokes Law which shows that the water settling rate decreases as the difference between the water and oil density narrows and as the viscosity of the oil increases. To some limit, heat can be applied to reduce the oil viscosity but this has little effect on the phase density difference. Heavy feedstocks, then, can be troublesome to desalt.
It is generally recognized that water-insoluble solids such as rust, iron sulfide, silt, clay, drilling mud components, etc., complicate the demulsification process. These materials tend to collect at the interface between the oil and water and contribute to emulsion stability. This material accumulates in a portion of the emulsion to form what is referred to as a cuff or rag layer. Emulsifiers used in drilling muds and surfactants used in tertiary production more than likely contribute to this problem.
It is desirable in desalting operations to produce an effluent water free of hydrocarbons--at least to the extent possible. This means that the cuff or rag layer is ultimately forced overhead in the desalted crude at the rate at which it is produced. This produces desalted crude of unacceptable quality which at least in part defeats the purpose of the desalting operation.
According to this invention heavy crude oils or their fractions, containing water-insoluble contaminants which are difficult to desalt because clean splits cannot be obtained due to a cuff layer emulsion which collects at the oil-water interface are processed by removing the cuff layer, recycling a major portion to the crude oil feed and drawing off a slip stream to avoid solids contaminant buildup. The slip stream is diluted with a light hydrocarbon material to break the emulsion and thereafter the oil reduced in solids content is recovered by settling and decantation. In one aspect of the invention the oil phase and cuff layer are removed together from the desalter and taken to a second vessel in which they are separated. The cuff layer is then processed as set forth above.